Tire pressure monitoring systems can be implemented as direct tire pressure monitoring systems or indirect tire pressure monitoring systems. Direct tire pressure monitoring systems (TPMS) employ pressure sensors mounted in or on each tire to physically measure the tire pressure in each tire. Indirect TPMS do not use physical pressure sensors, but instead indirectly measure tire pressure by monitoring other available signals such as individual wheel rotational speeds (e.g., detected by an automatic braking system wheel speed sensor).
In the United States, TPMS are required to measure tire pressure of individual tires within a tolerance of +/−20%. While indirect TPMS systems are reliable, they do not meet the 20% requirement. For example, an indirect TPMS using a rolling speed comparator wouldn't detected if all tires of an automobile are equally deflated due to a temperature change. Therefore, in the United States TPMS systems often comprise a tire pressure sensor in every tire with a battery and a transponder to send signals to an electronic control unit. Such systems are costly since they require a sensor in every tire and are service intensive since they are battery driven.